This invention relates to a preferred spindle device and particularly to a built-in motor type spindle device preferably used by a high-precision machine tool.
Conventionally, a general spindle device has its spindle supported to rotate by rolling bearings (such as ball bearings or roller bearings) or static pressure bearings which use static pressure of a working fluid such as oil or air.
The spindle device using rolling bearings causes no problem at lower speeds since balls or rollers rotate between inner and outer wheels, but causes problems of a spindle runout and heat generation at higher speeds since it causes vibration and frictional heat. To prevent this, it is necessary to lubricate the running wheel surfaces. This is time-consuming for maintenance and increases the running cost of the equipment.
Meanwhile, since the static pressure bearings support a shaft by a static pressure of a pressure transmission medium, the built-in motor type spindle device (see Patent Document 1) can be expected to have a higher rotational accuracy than the spindle device which uses rolling bearings. Therefore, the static pressure bearings are suitable for finishing machines which processes optical transfer surfaces of optical element molding dies to form optical surfaces of optical elements.
As for a high-precision working machine including the spindle device which uses static pressure bearings, the speed of the spindle is preferably increased to improve the working efficiency. However, when the spindle is rotated fast, shearing of the pressure transmission medium is caused in the static pressure clearances to which the medium is supplied (minute clearances between the static pad and the spindle). This causes generation of heat in the bearings. Further, the shearing resistance of the pressure transmission medium which is generated simultaneously consumes part of the driving force of a motor which drives the spindle. Particularly, after the spindle speed reaches a preset speed in the built-in motor type spindle device which requires very fast rotation, almost all additional power supplied to the motor is consumed as heat without increasing the speed. This heat will locally make the spindle hotter and unignorably deform the spindle. In extreme cases, the spindle will have a break such as galling and seizure. In other words, one of the most important problems to be solved in the spindle which is supported by static pressure bearings to rotate fast is to cool the spindle efficiently.
The static pressure pad and other part of a static pressure bearing can be cooled by providing a pipe and applying cooling water to them. Similarly, the core section of the motor can be cooled by blowing-out of air or circulation of cooling water. For example, a motor of the type disclosed by Patent Document 1 is equipped with a water jacket inside the motor housing with which the motor core is in contact to cool the motor by water which flows through the water jacket. However, this configuration does not assure effective heat radiation due to contact since the spindle supported by the static bearings is not in direct contact with the motor housing. Also in this case, only cooling of the motor housing is not enough to cool the spindle when the spindle becomes very hot due to heat generation of the motor coils and magnets by power supply.
[Patent Document 1] Japanese Patent Publication Tokkaihei 10-19043